That's assuming you are starting to develop the project from scratch. But that's not the case. Imagine, you join a company and their product is one, ugly monol

That's assuming you are starting to develop the project from scratch. But that's not the case. Imagine, you join a company and their product is one, ugly monolith with a lot of technical debt. Now, you need to build some big and complicated features on top of this, and there are no similar features so far. Implementing it as a service can be much faster, have quicker iterations (especially if the deployment methods for the monolith are crap, which is often the case) and easier to build a high quality implementation. As long as you don't go crazy and ended up with ridiculous number of poorly separated services, this might be a good decision in long term (been there, done that). Based on anecdotal data (where n=3), that's very often the case.